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Measuring Radiation Intensity
Exposure is a quantity describing how much ionization is produced
(i.e., how many electrons are liberated) in air by gamma- or x-rays. The
unit of exposure is the Roentgen or Coulombs/kg (in the SI system).
- 1 Roentgen = 2.58 x 10-4 Coulombs of charge produced
per kilogram
of air.
The quantity, absorbed dose, describes how much energy is deposited
in a material by a beam of radiation and is not restricted to x-rays or
gamma rays passing through air. The unit of absorbed dose is the rad or
Gray (in the SI system).
- 1 rad = 100 ergs of energy deposited in one gram of material
- 1 Gray = 100 rads
Different types of radiation may deposit the same amount of absorbed dose but
produce different effects and different levels of damage. For instance,
charged massive alpha particles will interact more intensely and deposit energy
over a shorter distance within a cell than uncharged massless gamma rays and
will be more effective at producing biological damage within that cell, even
though equivalent amounts of energy are deposited
overall.
The quantity, dose equivalent, is derived by multiplying the absorbed
dose by a quality factor (QF) which depends on the type of radiation being
measured. The unit of dose equivalent is the rem or Sievert (in
the SI system).
- Dose equivalent = Absorbed dose x QF
- QF = 1 for gamma rays, x-rays and most beta particles
- QF = 2 -11 for neutrons, depending on energy
- QF = 20 for alpha emitters under conditions of internal exposure
- 1 Sievert = 100 rems
Back to Radiation Units
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